1. Field of the Invention
The present invention relates generally to a sensor actuator bus and, more particularly, to an information communication system for use with a plurality of devices, such as sensors or actuators, which permits rapid communication of sensor status between devices connected in signal communication with the bus. U.S. patent application Ser. No. 07/993,180, which was filed on Dec. 18, 1992, by Sitte and assigned to the Assignee of the present application, relates to a sensor actuator bus such as that which is the subject of the present application.
2. Description of the Prior Art
When a plurality of sensors or actuators are associated together to provide a control system, such as an assembly line or other type of manufacturing equipment, which requires numerous sensing and actuating devices, each device could possibly be connected individually to a central controller such as a programmable logic controller (PLC). However, since two wires would be extended between each device and the PLC,the amount of wiring needed to accomplish this function becomes economically prohibitive if a large number of sensors and actuators are involved in the system. Since each sensor or actuator requires both the provision of electrical power and a means for transmitting signals to and from the device, it is conceivable that four wires could be used to connect each of the sensing or actuating devices to both a power supply and a central controller. However, as is known to those skilled in the art, several means have been developed in an attempt to avoid this costly approach.
U.S. Pat. No. 4,374,333, which issued to Avery on Feb. 15, 1983, discloses a two terminal Hall sensor which is incorporated in an integrated circuit. The integrated circuit also comprises a voltage regulator and a threshold detector. DC power is supplied to the circuit through a pair of terminals that may be connected to a remote DC voltage source. A switchable constant current sink circuit connected across the terminals is turned on and off in response to the two output states of the threshold detector that, in turn, is responsive to the Hall sensor output voltage. Thus, the DC current flowing in the two DC supply lines is a predictable and recognizable value corresponding to a low or high magnitude magnetic field at the sensor. The magnitudes of the current flowing in the two DC supply lines is therefore recognizable even when the DC supply voltage changes over a wide range.
U.S. Pat. No. 4,791,311, which issued to Vig on Dec. 13, 1988, discloses an integrated circuit sensor that is capable of being energized through two DC terminals of the integrated circuit. Each sensor, when connected in parallel so as to be energized from a single DC supply voltage source, is capable of recognizing a unique pulse signal code superimposed on the DC supply voltage. When the sensor recognizes its code on the two-wire bus, it powers up its transducer, such as a Hall element and responds by drawing a particular current pattern from the common DC voltage source which is indicative of the presence or absence of a magnetic field that is ambient to the sensor. Electrically controlled energy to the transducer is provided only when the unique code is present on the two-wire bus. The steady regulated output voltage supplies energy to the MOS logic including the address code comparator. The sensor quiescent current drawn from the DC voltage supply line is thereby caused to be a very low value which becomes increasingly advantageous as more and more sensors are operated in parallel on the same DC supply line.
U.S. Pat. No. 4,677,308, which issued to Wroblewski et al on Jun. 30, 1987, discloses a switch status monitoring system. The system described in the Wroblewski et al patent provides continuous status monitoring of a plurality of switches and smart sensors associated with the switches, wherein each sensor is connected to a separate single point on a single-wire bus. The monitoring is affected by a smart sensor multiplex arrangement. The arrangement employs a microcomputer and a driver and receiver circuit for developing a particular pulse train wave form which is placed on the bus to provide power and control voltage signals to the plurality of smart sensors. The smart sensors contain circuits that respond to the waveform in a manner that causes each smart sensor to send current signals back over the single wire bus to the driver and receiver circuit and the microcomputer during designated repetitive and sequential time slots. The driver and receiver circuit receives, interprets and converts the current signals into voltage signals used by the microcomputer for establishing a history of the status of the bus, the sensors and the switches. The micro computer supplies continuous and updated information to a display system indicative of the status of each sensor and its associated switch.
The Avery, Vig and Wroblewski et al patents, described immediately above, are three examples of communication systems which are associated with a two-wire bus, or a one-wire bus with a ground connection as described in the Wroblewski et al patent, which communicates status information from a plurality of sensors or switches connected to the bus. It should be understood that these two-wire communication systems are representative of many other such systems that are very well known to those skilled in the art. It should also be understood that these two-wire communication systems each exhibit a common disadvantage because each of these three systems requires that each device connected to the system by polled sequentially and that each device only communicate its status when it is polled. This type of communication system typically provides a series of pulses on the two-wire bus and each device connected to the bus counts the pulses until the total sum equals its assigned identification value. If one of the sensors changes state much more often than the other sensors, this type of communication system does not provide a way to accommodate this situation. The sensors cannot, on their own, initiate a transmission in which the sensor communicates its status or change of state to a central controller and, in addition, the central controller has no convenient way to request a status from one specific sensor without first interrogating each device which has a priority identification with a binary number less than the specific device whose status is to be interrogated.
Many different protocols have been developed to permit efficient communication between individual devices, such as sensing and actuating apparatus, and a central control device, such as a PLC. One communication protocol that is particularly well suited for use in systems of the type described above is the Controller Area Network (CAN) that has been developed by Robert Bosch GmbH. The details of the CAN protocol are described in the 1991 revision of the CAN specification, versions 1.2 and 2.0, which is explicitly incorporated by reference herein. The CAN protocol provides a serial communication system that efficiently supports distributed real time control of a plurality of devices with a very high level of security. It can be used in association with high speed networks or low cost multiplex wiring systems. In certain automotive applications, such as electronics, engine control units, sensors or anti-skid systems, the CAN protocol supports bit rates up to the 1 Mbit/sec while maintaining a cost effective method that can be incorporated into vehicle body electronics such as lamp clusters, electric windows, etc. without requiring complex wiring harnesses. Some applications incorporate a microprocessor in association with a CAN protocol chip and several sensors are usually connected in signal communication with the microprocessor. As an example, a single microprocessor can be associated with a pressure sensor, a temperature sensor and a photoelectric device. All three sensors would be connected to ports of the single microprocessor. In addition, the CAN protocol chip would be connected to both the microprocessor and the two-wire bus. The microprocessor would receive signals from the three sensors, determine the source of the signal and formulate a message for transmission by the CAN protocol chip to the two-wire bus.
The CAN protocol incorporates eleven bits that act as a priority code to identify a particular device connected to the two-wire bus. As is understood by those skilled in the art, the eleven priority code bits of the CAN protocol system can be used to identify a particular device in the system or, alternatively, can be used to identify a particular message. For example, in certain systems there might only be one temperature sensor. The priority bits in the CAN protocol could be used to identify a particular message that represents an overtemperature condition. In that case, there is no need to identify the device that caused the message to be generated since only one device could have generated it. On the other hand, if many identical sensors are connected to the system, the eleven priority bits would be used as an identifier field to specify particular sensors. A 12th bit in the bit stream operates as a Remote Transmit Request (RTR) which designates that a remote device should transmit a message. The following two bits are reserved for use by the CAN system. The CAN protocol next provides four bits that define the length of a following data field. The data field is limited to eight bytes, or sixty-four bits, of information.
A system incorporating a protocol such as the CAN protocol operates in a highly efficient and satisfactory manner for most systems, particularly when extremely high data rates are not required. More specifically, sensor and control systems in an automotive application are particularly well suited for use with the standard CAN protocol. However, certain applications can require rates of data exchange beyond those that can be easily accommodated by the CAN protocol. For example, certain industrial applications incorporate a very large number of sensors, such as limit switches, proximity sensors, photoelectric devices, etc.. In addition, an industrial system may incorporate a large number of actuators such as solenoids, air cylinders, etc.. The statuses of each of the sensors connected to a two-wire bus of this type in an industrial application can change randomly and at a very rapid rate. Since the CAN protocol requires that each device connected to the bus be given a priority value which can be used as the device identifier, the application of a large number of sensors and actuators to a bus can result in deleterious delays in communicating the status of low priority devices to a central controller, such as a PLC.
Although the CAN protocol provides a highly efficient and powerful method for communicating information between devices on a bus, it would be beneficial to the field of industrial automation if certain types of information could be communicated at a greater rate of transmission than is normally contemplated by the CAN protocol. This is particularly true in certain industrial automation systems which incorporate many devices, such as limit switches, proximity sensors and photoelectric devices, which can exhibit only one of two possible states (e.g. open/closed, on/off, present/absent). It would therefore be beneficial if devices such as these could transmit their status in a streamlined way that shortens the time needed for the transmission of data using the standard CAN protocol.